This invention relates to a process for making a lithographic printing plate, which involves the scanning exposure technique to achieve high-intensity short-time exposure and the silver complex diffusion transfer development process. It relates also to a plate-making and printing method using such a printing plate.
A lithographic printing plate which utilizes as ink receptive areas a transferred silver image produced by the silver complex diffusion transfer process has already been known to the art by the disclosure of Japanese Patent Publication No. 30,562/73 and Japanese Patent Application "Kokai" (Laid-open) Nos. 21,602/78, 103,104/79 and 9,750/81.
In a typical embodiment of the silver complex diffusion transfer process suitable for making a lithographic printing plate, a silver image is formed in the following way: When a photosensitive material comprising a support and, disposed successively thereon, a subbing layer which serves also as antihalation layer, a silver halide emulsion layer, and a physical development nuclei layer is imagewise exposed and developed, the silver halide in the areas where a latent image has been formed changes into developed silver in the emulsion layer. At the same time, the silver halide in the areas where a latent image has not been formed dissolves by the action of a silver halide complexing agent contained in the developer and diffuses toward the surface. The dissolved and diffused silver complex is precipitated by the reducing action of the developing agent on the physical development nuclei in the surface layer to form a silver image. After the development, if necessary, the silver image is subjected to sensitization treatment to enhance its ink receptivity. The material thus treated is mounted on an offset press and the inked image is transferred to a print material.
In the conventional process, the silver halide emulsion layer is spectrally sensitized with a merocyanine dye, cyanine dye, or the like so as to show a sensitivity maximum at around 550 nm in the green region. The emulsion layer is exposed in a process camera to a customary light source such as a tungsten bulb for several seconds to several tens of seconds. A printing plate obtained in such a way, however, exhibited only limited sharpness and resolving power, even though an aforementioned sensitive material inherently excellent in these properties has been used. Moreover, in reproducing a color print from a color original, the conventional process has disadvantages in that both procedures of preparing the sensitive material and making a printing plate are troublesome in addition to the insufficient resolving power.
A conceivable idea to overcome the above difficulties is that the photosensitive material is given a exposure by means of a scanning exposure technique; namely, the material is exposed momentarily (a period of 10.sup.-5 second or less) to high-intensity light sources including various lasers such as neon-helium laser, light-emitting diodes (LED) and cathode ray tubes (CRT), whereby direct printing plate making becomes possible. There have been disclosed in many patents those photographic materials which are sensitized with dyes suitable for lasers used as light source. Although realized in other fields, such photographic materials are not yet actually used in the field of lithographic printing plate which utilizes the silver complex diffusion transfer process. Among others, a technical reason for this is the difficulty of obtaining a silver halide emulsion exhibiting a sensitivity and a resolving power sufficiently high for practical use. It is known that a high-intensity momentary exposure gives rise to so-called high intensity reciprocity law failure, resulting in deterioration in both sensitivity and graduation. In addition, as compared with the common silver halide photosensitive materials, in the sensitive material for lithographic printing plate utilizing the silver complex diffusion transfer process, the physical development nuclei layer greatly affects the spectral sensitization and, as a consequence, there is resulted a decrease in the sensitivity to a laser exposure or deterioration in the shelf stability. Since the silver complex diffusion transfer process produces a transferred and precipitated silver image of soft gradation, there occur a decrease in both sharpness and resolution of the image, scumming, and worn-off of the silver image during printing. These defects result in insufficient printing endurance and detract much from the printing plate quality. A desirable sensitizing dye should meet the following requirements: sufficiently high sensitivity to the radiation wave-lengths from lasers; good shelf stability; formation of a transferred and precipitated silver image of high contrast; no adverse effect such as scumming; and formation of a transferred and precipitated silver image having a sufficient adherence to keep even small silver particles from worn-off during the printing operation.
As for the lasers, there are many types using various media such as ruby, neon-helium gas, argon gas, krypton gas, cadmium vapor, and gaseous carbon dioxide. Of these madia, the neon-helium gas is widely used because of its lowest cost and the steadiness of laser output. The radiation wave-length from a neon-helium laser is 632.8 nm. For instance, in making a lithographic printing plate by means of an electronic color scanner, the silver halide emulsion layer should have a high sensitivity to 632.8 nm radiation from the laser, in addition to other required properties mentioned above.
Silver halide photosensitive materials containing sensitizing dyes suitable for the neon-helium laser exposure are disclosed by Japanese Patent Publication Nos. 17,725/74, 39,818/80, and 15,011/80. These sensitizing dyes generally show sensitivity maxima in the range of from 625 to 660 nm which covers the wavelength, 632.8 nm, of neon-helium laser. Almost all of such dyes, however, are unsatisfactory in meeting all of the aforementioned requirements for the present lithographic printing plate. It was found that the sensitizing dyes given hereinafter can be satisfactorily used in the photosensitive material to be exposed to a laser beam.
The light-emitting diode (LED) is inferior in the output to the neon-helium laser. Consequently, the photosensitive material is required to be of higher sensitivity and contrast, excellent in sharpness and resolving power, and capable of producing a printing plate resistant to scumming and worn-off of the silver image. The sensitizing dye used in such a photosensitive material should have not only sufficiently high sensitivity to a wavelength range of beams from steadily utilizable LED's, i.e. about 660 to about 730 nm (the wavelength width of any particular LED radiation is about 15 nm) but also other required properties including good shelf stability, ability to form a high-contrast silver image, no adverse effect such as scumming, and capability of forming a silver image having sufficient adherence to keep even tiny silver particles from worn-off during the printing operation.
It is described in U.S. Pat. No. 4,134,769 that when a betain- or anion-type cyanine dye is used, there is obtained an offset printing plate having a high sensitivity and improved printing characteristics and that to obtain a high sensitivity, it is desirable to use a silver halide emulsion of a high silver bromide content. On the other hand, it is known that in the case of a high-intensity short-time exposure, a silver halide emulsion of high silver chloride content is inferior in spectral sensitization effect to that of a low silver chloride content (e.g. Japanese Patent Publication No. 42,172/73); it is also known that such an emulsion is more susceptible to high-intensity reciprocity law failure [e.g. Photographic Science and Engineering, Vol. 26 (1982)]. It may be said, therefore, that an emulsion of high silver chloride content is unsuitable for a scanning exposure using a laser or LED as light source.